add loess selectivity / ogive

[smormede]

The preferred methods in some other assessments. Would be good to include in casal2 at a later date or as the need arises

[Craig44]

Future source: https://kluge.in-chemnitz.de/opensource/spline/

https://github.com/ttk592/spline/blob/master/src/spline.h

[Casal2]

Closing issue. Been a few years and will be placed on a roadmap if required.